Topological insulators:


Three HgTe wires made by ebeam lithography and wet chemical etching from HgTe

Three-dimensional (3D) topological insulators (TI) feature conducting 2D surface states while the interior of the material is insulating (or semiconducting). In such a 3D TI a two-dimensional electron system is "wrapped" around the bulk. We mainly focus on mercury-telluride (HgTe) based materials which have a very high charge carrier mobility allowing to probe quantum and ballistic effects. In the past we have analyzed this unique two-dimensional electron systems by combined (quantum) transport and (quantum) capacitance experiments which allow to disentangle contributions from top and bottom topological surface. Recently, we focus on nanopatterned HgTe films, i.e. nanowire geometries and antidot arrays to probe the peculiar enery spectrum of one-dimensional topological systems and to explore ballistic effects in this new class of materials


Antidot array etched into the surface of a 80 nm thick HgTe layer (bright layer)

Recent References
  1. J. Ziegler, R. Kozlovsky, C. Gorini, M.-H. Liu, S. Weishäupl, H. Maier, R. Fischer, D.A. Kozlov, Z.D. Kvon, N.N. Mikhailov, S.A. Dvoretsky, K. Richter, D. Weiss, Probing Spin Helical Surface States in Topological HgTe Nanowires, arXiv:1708.07014, (2017).
  2. H. Maier, J. Ziegler, R. Fischer, D.A. Kozlov, Z.D. Kvon, N.N. Mikhailov, S.A. Dvoretsky, D. Weiss, Ballistic geometric resistance resonances in a single surface of a topological insulator, arXiv:11708.07766, (2017).
  3. D.A. Kozlov, D. Bauer, J. Ziegler, R. Fischer, M.L. Savchenko, Z.D. Kvon, N.N. Mikhailov, S.A. Dvoretsky, D. Weiss, Probing Quantum Capacitance in a 3D Topological Insulator. Phys. Rev. Lett. 116, 166802 (2016).
  4. D.A. Kozlov, Z.D. Kvon, E.B. Olshanetsky, N.N. Mikhailov, S.A. Dvoretsky, D. Weiss, Transport Properties of a 3D Topological Insulator based on a Strained High-Mobility HgTe Film. Phys. Rev. Lett. 112, 196801 (2014).

All electrical spin injection and detection


Nonlocal configuration, with S being an injector and D a detector in (a) 4-terminal and (b,c) 2-terminal geometry. Graphy below show the corresponding spin-accumulation (see [1])

Using both the electrons' charge and spin is at the heart of spintronics. The-Datta-Das spin-transistor is a paradigmatic but so far experimentally not realized example of the underlying concepts. We explore all electrical spin injection and detection in fully epitaxial semiconductor heterostructures. As ferromagnet we use the ferromagnetic semiconductor (Ga,Mn)As which features a high spin polarization at the Fermi level. By driving an electrical current from a ferromagnet into a semiconductor based two-dimensional electron system a spin polarized current (two-terminal geometry) or a pure spin current (in four-terminal geometry) can be generated. Recently, we showed a significant enhancement of non-local spin signals if spin injection takes place into a quasi-ballistic two-dimensional electron gas. By biasing the ferromagnetic detector the detection sensitivity can be enhanced significantly enabling large spin signals DR/R ~80% in conventional spin valve (two-terminal geometry) at cryogenic temperatures. Furthermore, gates outside the current path can tune the spin-signal by a factor of up to 6 providing an alternative means for realizing transistor like devices.

Colored electron micrograph showing a spin-transistor like device with two ferromagnetic contacts (red) and two gates outside the current path (dark) (see [1])

Recent References
  1. Martin Oltscher, Franz Eberle, Thomas Kuczmik, Andreas Bayer, Dieter Schuh, Dominique Bougeard, Mariusz Ciorga, Dieter Weiss, Gate-tunable large magnetoresistance in an all-semiconductor spin-transistor-like device, arXiv:1706.05239 (2017)
  2. T. Kuczmik, M. Oltscher, A. Bayer, D. Schuh, D. Bougeard, M. Ciorga, and D. Weiss, Hanle spin precession in a two-dimensional electron system, Phys. Rev. B 95, 195315
  3. M. Oltscher, M. Ciorga, M. Utz, D. Schuh, D. Bougeard, and D. Weiss, Electrical spin injection into high mobility 2D systems, Phys. Rev. Lett. 113, 236602 (2014)